This invention relates to the measurement of dissolved gases in liquid samples by means of an amperometric sensor. Such sensors, known collectively as Clark cells, have been extensively described (Janata, J. Principles of Chemical Sensors, Plenum Publishing, 1991, Chapter 4; Polarographic Oxygen Sensors Gnaiger, E, and Forstner, H. (eds.) Springer-Verlag, 1983) and are widely used for the analysis of gases which are readily reduced or oxidized such as hydrogen sulfide, NO, CO, or oxygen. Clark cells consist of a gas permeable membrane enclosing an electrolyte in contact with a working and reference electrode. The gas crosses the membrane by diffusion, and is reduced or oxidized at the working electrode thereby creating a current flow. The stability and reliability of Clark cells depends on many factors. Of importance to this invention is the limitation on sensor lifetime imposed by the amount of electrolyte within the Clark cell. Since the oxidation and reduction processes consume components of the electrolyte, Clark cells are inherently prone to instability and limited lifetime due to the exhaustion of the electrolyte. This problem is particularly acute for small electrodes (microelectrodes). There have been many attempts in the past to address this shortcoming through mechanical means, for example European Patent No. EP0496521 proposes such a method. In addition, Clark cells are expensive to construct and require frequent maintenance and calibration.
A practitioner skilled in the art will recognize that exhaustion of the electrolyte is a serious barrier to the construction of long lived and stable Clark cells. The invention describes a membrane with a combination of properties: selective permeability for the gas to be analyzed and an ion exchange capacity to allow discharge of the ionic byproducts of the redox reaction. A membrane formed in this manner will allow an amperometric sensor to function in a stable manner for a much longer period of time than a sensor that must rely on only the ions present in the original electrolyte volume. This in turn will allow someone skilled in the art to produce a very small AND long lived, stable sensor.
Ion exchangers for both cations and anions are well-known, either formulated as a functionalized polymer membrane (see for example Kesting, R. Synthetic Polymer Membranes, 2nd Ed, Wiley-Interscience, 1985) or as a liquid membrane containing an ion exchanger (see for example Ion Exchange Processes : Advances and Applications, A. Dyer, M. J. Hudson, P. A. Williams.(Eds.) Royal Society of Chemistry, 1993). Both types facilitate the transfer of an ion across the membrane in exchange for a counter-flow of another ion. For example, quaternary ammonium salts in liquid membranes will facilitate the exchange of chloride ions flowing in one direction across the membrane with a flow of bromide ions moving in the opposite direction.
For the purposes of this invention, the membrane can be formed in a number of ways. One way, is as a supported liquid membrane (SLM) (see Liquid Membranes: Theory and applications, Noble, R. D and Way, J. D (Eds.) ACS Symposium Series 347, American Chemical Society, 1987). In such membranes a porous support polymer contains a solvent imbibed in the pores. An ion-exchanger is dissolved in the solvent thereby creating the SLM. A related SLM is a solvent-polymer membrane in which the solvent plus ion exchanger acts as a plasticizer for a polymer. For example a poly(vinyl chloride) film containing a phthalate plasticizer and an ion exchanger can create an ion exchange membrane. A third type of ion exchange membrane can be formed by modification of a polymer backbone to incorporate ion exchange groups directly linked to the polymer. Films of such materials can be cast or fabricated in a form suitable for this invention.